Constraint-based real-time scheduling for process control
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This research addresses real-time task scheduling in industrial process control. It includes a constraint-based scheduler which is based on MSP.RTL, a tool for real-time multiprocessor scheduling problems with a wide variety of timing constraints. This dissertation extends previous work in two broad directions: improving the tool itself and broadening the application domain of the tool to include wired and wireless industrial process control. For the tool itself, we propose enhancements to MSP.RTL in three steps. In the first step, we modify the data structure for representing the temporal constraint graph and cutting the memory usage in half. In the second step, we model the search problem as a constraint satisfaction problem (CSP) and utilize backmarking and conflict-directed backjumping to speed up the search process. In the third step, we perform the search from the perspective of constraint satisfaction programming. As a result, we are able to use existing CSP techniques efficiently, such as look ahead, backjumping and consistency checking. Compared to the various ad hoc heuristics used in the original version, the new approach is more systematic and powerful. To exercise the new MSP.RTL tool, we acquired an updated version of the Boeing 777 Integrated Airplane Information Management System(AIMS). This new benchmark problem is more complicated than the old one used in the original tool in that data communications are described in messages and a message can have multiple senders and receivers. The new MSP.RTL tool successfully solved the new benchmark problem, whereas the old tool would not be able to do so. In order to apply real-time scheduling in industrial process control, we carry out our research in two directions. First, we apply the improved tool to traditional wired process control. The tool has been successfully applied to solve the block assignment problem in Fieldbus networks, where each block comprising the control system is assigned to a specific device such that certain metrics of the system can be optimized. Wireless industrial control has received a lot of attention recently. We experimented with the tool to schedule communications on a simulated wireless industrial network. In order to integrate the scheduler in real wireless process control systems, we are building an experimental platform based on the WirelessHART standard. WirelessHART, as the first open wireless standard for process control, defines a time synchronized MAC layer, which is ideal for real time process control. We have successfully implemented a prototype WirelessHART stack on Freescale JM128 toolkits and built some demo applications on top of it. Even with the scheduler tool to regulate communications in a wireless process control, it may still be possible that communications cannot be established on an inferior wireless link within an expected period. In order to handle this type of failures, we propose to make the control modules aware of the unreliability of wireless links, that is, to make the control modules adapt to the varying link qualities. PID(Proportional, Integral, Derivative) modules are the most used control modules. We developed PIDPlus, an enhanced PID algorithm to cope with possible lost inputs and outputs. It has been shown that PIDPlus can drastically improve the stability of the control loop in cases of unreliable wireless communications.